After drilling a wellbore in the oil and gas industry, the drilled wellbore can be subsequently completed by cementing a string of metal pipes connected end-to-end within the wellbore. Commonly called “casing,” such pipes increase the integrity of the wellbore and provide a flow path between the earth's surface and selected subterranean formations. Some wellbores are lined with multiple concentrically-positioned pipes (i.e., concentric casings). Moreover, in some wellbores, one or more production pipes are extended into the wellbore to provide a conduit for hydrocarbons to be conveyed to the earth's surface. Accordingly, as used herein, the term “pipe” or “wellbore pipe” will refer to metal pipes or pipelines that line the walls of a wellbore, such as casing, and also production pipes extended into a wellbore to facilitate hydrocarbon production operations.
During the lifetime of a well, wellbore pipes are exposed to high volumes of materials and fluids required to pass through them, including chemically aggressive fluids. In harsh environments, the pipes may be subject to corrosion that may affect their functionality. Consequently, the structural integrity of wellbore pipes may change over time due to chemical and mechanical interactions. Moreover, due to the length, volume, accessibility difficulties, and long time periods associated with the process, it is a costly task to monitor wellbore pipes and pipelines and intervene when required. For instance, this may be a difficult task for pipes that are commonly used for the extraction of hydrocarbons, since such pipes and pipeline investigation systems are required to be exposed to elevated pressures and temperatures.
The diameter of the innermost pipe of concentric wellbore pipes limits the size of the monitoring and intervention system that can be deployed to monitor the integrity of all of the concentric pipes. Moreover, effectively monitoring the outermost pipes from the innermost pipe may be difficult since any monitoring system has to be able to sense through a number of pipe layers, each of which may have developed distinct problems or defects. In some wells, the geometry of the well can be more complicated where two eccentric production pipes are positioned inside a wellbore pipe that is concentrically arranged within multiple additional pipes. In this situation, the pipe inspection tool may be required to log from the interior of one of the production pipes and extract information from all concentric and eccentric wellbore pipes.
In conventional pipe inspection tools, the length of a coil antenna included in the pipe inspection tool controls the radial depth at which the metal thickness of the surrounding pipes can be measured. A longer coil antenna can reach a deeper radial distance, but the magnetic field created by that coil antenna spreads over a larger volume, and naturally decreases the resolution of the longer coil antenna.